One type of power metal oxide semiconductor (MOS) field effect transistor is the lateral double diffused metal oxide semiconductor (DMOS) field effect transistor. Generally, as a measure to improve the breakdown voltage of this type of DMOS field effect transistor, the length of the drift region (drift length) is extended to improve the breakdown voltage of an element. However, when the drift length is increased, there is a problem that the element area of the lateral DMOS field effect transistor is increased.
Also, in a DMOS field effect transistor, in order to make element failure due to avalanche breakdown difficult to occur, generally a back gate region with a conductivity type opposite to that of the source region is provided at the source region, as a measure to remove carriers generated by avalanche breakdown from the back gate region.
It is desirable that all source regions is provided with a back gate region, in order to effectively remove carriers generated by avalanche breakdown from the back gate region. However, if a back gate region is provided at all source regions, the element length A (distance between source and drain) becomes longer, and the on resistance (RonA) between the source and drain increases, and this has a problem that the element area increases.
A measure against this is to provide a source region without a back gate region in parts, to suppress the increase in element area and reduce the RonA. However, in this case, there is the issue that when avalanche breakdown occurs at the element region without a back gate region, there is a greater danger of element failure.